The field of the invention is that of cooled optical mirrors.
In the field of cooled mirrors for high-power lasers and optical systems, the state of the art has changed from copper mirrors to molybdenum and from single-pass cooling to multiple layers of coolant channels.
Prior art mirrors have been fabricated with faceplate, heat exchangers and substrate of the same material for the obvious reason that differential expansion increases the distortion of the mirrors. The change from copper to molybdenum produced a better compromise between a high coefficient of thermal conductivity and a low coefficient of thermal expansion, at the cost of tackling the problems of working with molybdenum.
Work with different forms of heat exchangers has shown that state of the art molybdenum mirrors offer rapidly diminishing returns in that faceplate distortion decreases only slowly as cooling capacity increases. It is evident to those skilled in the art that future requirements for high flux, highly cooled mirrors that have extremely low distortion cannot be met by further refinements of present molybdenum mirrors. For the particular case of a laser operating in space, where the distortion requirements are more severe than for typical earth-based applications and where there is an added weight requirement, molybdenum mirrors are even less capable of meeting the system requirements.
Those skilled in the art have long sought an improved material for high-power mirrors, one approach being that disclosed in U.S. Pat. Nos. 4,142,006 and 4,214,818 issued to W. J. Choyke and R. A. Hoffman, showing the use of a hot-pressed silicon carbide mirror, in which the optical surface is either polished onto the substrate or is polished onto a vapor deposited layer of silicon carbide. These mirrors have the advantage that using the same material for the substrate and the faceplate reduced optical distortions caused by dissimilar thermal expansion. They also have the advantage that silicon carbide has less than one-third the density of molybdenum. These mirrors suffer a disadvantage, however, in that the thermal conductivity of silicon carbide is less than molybdenum.
The invention relates to a lightweight mirror for reflecting high power optical beams comprising a reaction sintered silicon carbide substrate supporting a faceplate formed by one or more plates of silicon containing coolant passages therein.